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<head>
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    <title>Document</title>
<style>
    /* li:not(:first-child){
        background: #000;
    } */
    li:not(:nth-child(2n)){
        background: #000;
    }
</style>
</head>
<body>
    <ul>
        <li>111</li>
        <li>222</li>
        <li>333</li>
      <li>444</li>
        <li>555</li>
    </ul>
</body>
</html>